I am a member of Goodreads already. I was thinking more in an academic sense, more in the sense of pursuing academic questions and seeking answers via book purchases/loans etc, downloading Papers etc.
GR in another sense functions like a Reading List but I am after finding others who pursue similar aims, other autodidacts who go after what they want in the face of incredible and overwhelming obstacles; in order to answer difficult and pressing questions.

The library is the best place to start I think since books tend to be organized in topics instead of by author or title. I like to go to the catalog, find one book which might be helpful, find it on the shelf, and then look at all the books around it. It's never let me down! :)

The way I build my book lists is by first researching the topic on the internet - reading articles, discussions, forums, etc. Often there are references to books either in the posts themselves or the comments that follow it. I then search for these books on Amazon - there I can find related products, references to other products in people's reviews, check out a sample, and see lists compiled by others interested in the same topic.
I don't always buy my books through Amazon (if I can get it at the local library that's ideal!), but I do find it useful in sifting through the multitude of options.